Method of recovering solids from gases



y 1951 :w.' F. ROLLMAN 2,550,722

METHOD OF RECOVERING SOLIDS FROM GASES Filed July 10, 1947 2Sheets-Sheet 1 i //7Ve/7 for DDLM/WM/ ey y 1951 w. F. ROLLMAN 2,550,7222 METHOD OF RECOVERING SOLIDS FROM GASES Filed July 10, 1947 2Sheets-Sheet 2 wcwpezzv [IVER-r SOL I GAS 6'45 FIG-'5 r76: 6

Patented May 1, 1951 GASES Walter F. Bellman, Cranford, N. J., assignorto Standard Gil Development Company, a corporation of DelawareApplication July 10, 1947, Serial No. 760,143

The present applicationis a continuation-in- :partnimy abandonedapplication Serial No. 532,-

1.79, filed April 21, lit-l4.

'My. invention relates to the. novel features hereinafter fullydisclosed in the present specification and claims, and more particularlyit relates to the-recovery of volatile solids from -a gas. V

A specific embodiment of my invention involves the recovery of phthalicanhydridein vaporxfrom. a gas stream. Phthalic onhydride is made b thecatalytic vapor phase oxidation of naphthalene, and 1 recentdevelopments have shown that it vcan'also be produced in a similarmanner from ortho-uylene. One diiiiculty with either process is that itis diincult to remove .the: oxidized product from the gas stream exitingfrom the reaction zone. Phthalic anhydride is present as a vapor in verylow concentration (lessthan 1%) in the hot gases issuin from thereaction zone, and-..appreciah1 time and large surface areas arenecessary to deposit the product as'a solid from the cooled gases.Ordinarily, the product gases are allowed to cool in large chambers fromwhich the crystalline product is periodically removed. This method iscumbersome, intermittent and does not result in complete recovery of theproduct from the gas stream.

The main object of my present invention has to do with the improvementsin the recovery of s0lids'from.the gases issuing from a reaction zone,in an expeditious and economical manner.

A specific object of my invention is to recover vaporized phthalicanhydride present in low concentration in hot gases under conditionssuch that the deposition of phthalic anhydride on cooling surfaces andthe like is substantially completely obviated.

Briefly, my method consists of passing the product gases from thereaction zone through a heat exchanger containing an inert granularsolid, in such away that the movement of the gases causes thecirculation of the solid in theexchanger, but does not cause said solidto be entrained in substantial amounts in the gases issuing from theexchanger. The product gases are thus cooled and the phthalic anhydrideis deposited on particles of the circulating solids. The scourin actionof the lDEItEOIldillfitBllEtl prevents the accumulation of the phthalicanhydride on contacting heat exchanger surfaces, or on the inert solidmaterial itself, and ultimatelyl finely di- :vided particles ofanhydride are carried out of the exchanger with the vent gases fromwhich 8 Claims. (c1. 183--11) they aresubsequently separated in acyclone sen .arator or other suitable means.

.zIn the accompanying drawing, I have shown in, Fig. 1 diagrammaticallyand in partial crosssection a structure which illustrates one form. of aheat exchanger or cooler in which hot gases containing phthalicanhydride mayb cooled; in Fig.2 I have shown a second modification of aheat exchanger or cooler difiering from Fig. 1 in that banks of coolingtubes are horizontally disposed within the heat exchanger; in Fig. :3 Ihave shown a heat exchanger consisting essentially'of three compartmentsor stages of cooling in which the preferred modification of my inventionmay be carried into efiect; and in .Figs. 4 to 8 inclusive Ihave shownfive related modifications in which a fluid coolant is caused to flow ina coil disposed in a fluidized bed of the solids, but in som of whichprovision is made to prevent freezing of the condensible solid enteringthe cooler, and plugging thereof, at the 'inlet.

Similar reference characters refer to similar parts throughoutthe'several views.

Referring in detail to the drawin s,.Fig.:1

shows a heat exchanger case I in which is disposed concentrically a flueor pipe 3 disposed with its lower end spaced above the bottom portion ofcase land extending to a point substantially below the upper portion ofthe said case, thus providin means for internal circulation of a, solid,as will subsequently appear.

Within the concentric tube or line 3 there is disposed a powdered solidmaterial 4, such as, say, powdered quartz, sand, pumice or the likehaving a particle size offrom 20-50 mesh. This material is mainta'med ina fluidized condition by causing a stream of hot gases containingpl'ithalic. anhydride to be injected.v throughv an 'educter I0 into thelower portion of the'fiue'fi where it flows upwardly at a net velocityof, say,

.15-20 ft. per second. 'I'hesolid powdered mathe lower space it and issubsequently injected into the flue 3 for further use in the process.The circulation of solid continues in the manner indicated.

In the process which I have described, the vaporized phthalic anhydrideis cooled below its solidification point and is withdrawn overheadthrough line 20 with the exit gases whereupon it may then be passedthrough a cyclone separator, or several of them, and/or through one ormore Cottrell precipitators or filters, in order to separate thephthalic anhydride crystals or flakes from the gases. I deem itunnecessar to include a showing of the cyclone separators, Cottrellprecipitators, or filters, since their mode of operation and structureare well-known to those familiar with the art, and for simplicitytherefore, I have not included a showing of them in the drawing.

In Fig. 2, the only substantial difference between this structure andthat of Fig. 1, is that the cooling tubes l5-a are disposed laterally inclose proximity to the flue 3 and that otherwise the operation of thedevice is substantially the same as that described in connection withFig. 1.

In Fig. 3, I have disclosed an enlarged heat exchanger having threestages of cooling. Structurally each individual stage .or compartment issimilar to that shown in Fig. l, but here I am enabled to obtain gradualcooling first with air or some other gas in the lower compartment, nextwith cooling water or steam in the intermediate compartment, and a finaldegree of cooling with brine solution in the upper compartment. Thecompartments are separated by separating walls 30,- but areintercommunicating by means of the eductors l6a and [-D projectingthrough the said separating walls so that flow of gases and/or vapors inseries through the system is permissible.

Further describing the apparatus shown in Figs. l'to 3 it is statedthat, in general, the eductor .10 must be at the extreme bottom of theflue or dryer to prevent precooling of the gases or vapors and hencepluggingof the inlet line with deposited solid. The incoming gas orvapor is instantly chilled by contact with the granular solid 4 and byindirect heat exchange with the coolant as it rises through the tubes 3.Phthalic anhydride condenses out of the gases as a solid, but the motionof the solid particles prevents the deposition of the solid on the tubeWalls, or on I the solid particles, and also prevents excessiveagglomeration of the particles themselves. The particles of the inertsolid carried to the space 5 ultimately gravitate to the bottom of theheat exchanger where they are rechilled and again areiniected into thefiues and the cycle isrepeated. Any inert solid may be used, provided itis reasonably hard and dense and does not dust or disintegrate readily.The velocity of the gas in the disengaging space is so adjusted that thephthalic'anhydride dust is carried overhead with the gas, but the inertsolid: settles out. Of course, to accomplish this, the solid particlesof the inert solid must be of larger size than the phthalic anhydrideparticles, or other solids, or if the inert particles are not largertheir densities must be greater than the material to be separated from agasiform medium.

The modifications shown in Figures 4 to 8, inclusive, are quite similarand may be described together. Referring, therefore, in detail toFigures 4 to 8, I00 represents a cooler or heat exchanger containing abody of fluidized powdered material. In each case, a gas containing anormally solid material enters at the bottom through posited within thecooler S00.

line llll. The condensible material contacts the powdered solids andforms therewith a .dense, fluidized mass having an upper dense phaselevel at L. There is also disposed in each of the reactors a coil I03through which a fluid coolant is caused to circulate for the purpose ofwithdrawing or abstracting heat from the solids, thus cooling thelatter, and the dense, turbulent bed of solids thus cooled lowers thetemperature of the incoming gas so that condensible material is de- InFigure 4, the

4 entering gasiform material to be cooled passes,

as shown, into the conical bottom and then passes upwardly, preferablythrough a screen or grid G, and contacts the solids, causingcondensation of the material to be solidified. The carrying gas iswithdrawn overhead through line I04,

in all modifications, with the entrained solid material which iscondensed in the cooler.

In all cases, the upper dense phase level L will, as usual, be fixed byregulating the gas velocities somewhere within the limits of to 2-3 feetper second, and also by fixing the actual number of pounds of solidsthat are in the cooler; and, as usual, above the dense phase level therewill be a dilute phase of solids sharply decreased in concentrationtoward the outlet of the cooler. It is also usual to dispose solid-gasseparating devices in the upper portion of such vessels to remove theinert solids and return them to the dilute phase. These details have notbeen described in full herein because they are by now well understood inthe art.

In Figure 5, provision is made for introducing a stripping gas throughlines 106. This gas may be added for the purpose of keeping thefluidized mass of solids well mixed and agitated because, of course,there is some danger that a sufiiciently low temperature will prevail tocause condensation at the inlet to the cooler or in close proximitythereto. If this eventuates, it may cause plugging in this region.

In Figure 6, I have shown burners lll'l for the purpose of adding heatto overcome any tendency for freezing or solidification of thecondensible solids in this region.

In Figure 7, I have shown an inverted cone type of inlet for the gasstream, and here again the purpose is to prevent plugging orprecondensation in the inlet region.

In Figure 8, which is quite similar to Figure 4, provision is made forcausing circulation of the solids well into the conical lower section tokeep the solids in this area well agitated.

As previously pointed out, my process involves recovering from agasiform material, a solid which is present therein, in particular wherethe solid'is present in small quantities and where the gas streamcontaining the normally solid material is at a sufiiciently hightemperature that the solid is in vapor or gaseous state. Normally, thesolid is recovered from the gas or vapor by cooling below itssolidification point, but if the gas is at a high temperature, it willbe readily appreciated that cooling sufiiciently to cause solidificationof the contained solid requires a large cooling surface area. Accordingto my process, I contact the hot gas containing the normally solidmaterial with a cooled solid material whereby the temperature of thegaseousstream is lowered sufficiently to cause solidification of thecontained normally solid material. The cooling solid is preferably inthe form of a fluidized mass of particles of appreciable size, whichsolid material moves continuously through an eduction zone or itsequivalent wherein the gaseous stream is cooled sufliciently to causedeposition of the contained solid on the cooling solid and thereafterthe desired solid, removed from the particles of cooling solid byfriction between the particles as a natural result of their rapid,churning motion through the vessel, is separated from the cooling solidin a disengaging space or zone and carried overhead in the original gasstream which is now cooled in the form of a suspension, while thecooling solid separates from the gas stream and disengaging space bygravity, is cooled and returned to the eduction zone for further use inthe process.

It is obvious that the precise details which I have enumerated above aremerely illustrative of my invention and many modifications fallingwithin the spirit of my invention will be apparent to those familiarwith this art.

What I claim is:

1. A continuous process for separating a normally solid materialexisting in vaporized form in a hot gasiform material stream whichcomprises charging the said stream into an eduction zone where itcontacts a cooled inert solid material in subdivided form, causing thegasiform material and the solid material to flow concurrently upward insaid eduction zone whereby the normally solid material contained in thegas stream is cooled below its solidification temperature, Withdrawingthe mixture of solid inert material and gaseous stream from the eductionzone and charging it into a disengaging zone where the linear velocityof the gaseous material is lowered sufliciently to permit the inertmaterial to settle out of the gas stream by gravity but sufficientlyhigh to maintain the solidified material in suspension, withdrawing thesolidified material in suspension from the disengaging zone for productrecovery, causing the separated solid inert material to flow through acooling zone wherein heat is abstracted therefrom, and thereafterreturning the cooled inert solid material to the eduction zone forfurther use in the process.

2. The method set forth in claim 1 in which the solid inert material hasa particle size of from 20-50 mesh.

3. The method set forth in claim 1 in which the normally solid materialcontained in the hot gaseous material entering the eduction zone isvaporized phthalic anhydride present in amounts of substantially 1% byvolume of the gas stream.

4. A continuous method of cooling a gas stream containing a minorquantity of vaporized phthalic anhydride which comprises dischargingsaid gas stream into the first of a series of intercommunicating coolingstages, contacting the said gas stream with a circulating granular solidinert material in the first of said stages, cooled by atmospheric air,whereupon some of the phthali o anhydride is condensed as a finelydivided solid suspended in the gas stream, separating the granularmaterial from the gasiform material in the first stage by reducing thevelocity of the gas stream sufliciently to permit settling by gravity ofthe said granular material but not the phthalic anhydride, causinggasiform material with entrained phthalic anhydride crystals to flowinto a second cooling stage where it contacts a sec ond portion ofgranular material circulating in heat exchange relationship with coolingwater whereby a further quantity of heat is abstracted therefrom andadditional phthalic anhydride iscondensed, reducing the velocity of thegasiform material in the upper portion of said second stage sufiicientlyto permit settling by gravity of the granular material but not entrainedproduct from the said gasiform material, conducting the gasiformmaterial into a third cooling stage where it contacts a third portion ofcirculating granular material circulating in heat exchange relationshipwith a brine solution whereupon the gas stream is cooled to atemperature such that substantially all remaining phthalic anhydride iscondensed, reducing the velocity of the gasiform material in a thirddisengaging stage to permit settling by gravity of the said granularmaterial but not entrained product, and withdrawing the gasiformmaterial together with entrained product in the upper portion of saidthird stage.

5. The method set forth in claim 4 in which the solid inert material andthe gas stream flow concurrently upward during the several coolingstages.

6. The method set forth in claim 4 in which the particle size of thesolid inert material is from 20-50 mesh.

'7. The method set forth in claim 1 in which the normally solid materialcontained in the hot gasiform stream entering the eduction zone has amelting point above the boiling point of water.

8. The method of recovering finely divided solid particles of a vaporousmaterial condensible to a solid from a gaseous mixture containing saidvaporous material, which comprises forcing the gaseous mixture upwardlythrough a fluidized mass of solid particles relatively larger in sizethan the finely divided particles of the vaporous material to berecovered, maintaining said fluidized mass of solid particles in acooling zone, cooling said fluidized mass of solid particles by heatexchange with a cooling fluid to below the solidification temperature ofsaid vaporous material so that said vaporous material contained in thegaseous mixture passing through said fluidized mass is condensed tosolid particles which are finely divided in said fluidized mass, andremoving from said cooling zone a gaseous stream containing entrainedfinely divided solid particles condensed from said vaporous materialwhile the relatively larger solid particles are kept circulating in thecooling zone.

' WALTER F. ROLLMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,064,468 Foster Dec. 15, 19362,393,636 Johnson Jan. 29, 1946 2,393,909 Johnson Jan. 29, 19462,448,135 Vecker et a1. Aug. 31, 1948

